Using sonar to describe temporal patterns of oceanic organisms from the MARS observatory

Abstract

DEIMOS (Deepwater Echo Integrating Marine Observatory System) is an active acoustic package deployed at the MARS cabled observatory in Monterey Bay, CA. Located on the continental slope at a depth of 875 m, DEIMOS transmits an acoustic pulse upwards every five seconds, recording the intensity of reflected sound (i.e. backscatter) from organisms in the water column. Designed as a robust, ROV-deployable package, it is built around a Simrad EK60 38 kHz scientific echosounder. The 7°, split-beam transducer is oil-filled to withstand pressure at depth. Electronics, including the general-purpose transceiver, an addressable web relay for remote rebooting, and a power filter and step-down, are housed in a 17-inch Teledyne Benthos borosilicate glass pressure sphere. The entire assembly is bolted to a galvanized steel frame and is attached to the MARS node by a 50-m Falmat Ethernet cable with an ODI wetmate connector. DEIMOS was deployed in February 2009 by the ROV Ventana from the R/V Point Lobos. Supplied with power and a communications link from the MARS node, DEIMOS is controlled remotely, and yields near-real-time backscatter data from the seabed to the surface. Since deployment, we have implemented automated data handling to monitor data acquisition, issue email alerts if it stops, and to upload data from the acquisition servers at Monterey Bay Aquarium Research Institute (MBARI) to the Fisheries Acoustics Research Lab in Seattle. Monterey Bay is located in the eastern-boundary California Current. Its food web depends on nutrients supplied by episodic, seasonal upwelling events, and its pelagic fauna are dominated by invertebrates. A variety of biological patterns are visible in the acoustic record. Several backscattering layers undergo diel vertical migrations, and change in number, depth, and density with the seasons. Episodic events such as high-density aggregations and large surface-diving targets are also present. We are developing metrics to characterize temporally-indexed, vertical density distributions. Total backscatter density increased from late spring to early autumn, while simultaneously becoming more dispersed and moving deeper in the water column. These changes reflect a decrease in the number of vertically migrating layers, and the appearance of a large, deep, non-migratory layer centered near 500 m depth. Late winter and spring contained short-term variability, due to the presence of near-surface aggregations. Long-term deployments of stationary acoustic instruments provide high resolution data over a range of temporal scales that is rare for aquatic biological research.